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I

QUANTITATIVE  ESTIMATION                   17                 '.,:

_                  * _

air expelled, for the gas then pushes the column of air before
it like a piston, before the latter has time to diffuse.    In about
ten  minutes, the  row  of burners beneath the spiral and the
coarse oxide to within 10 cm. (4 in.) of the fine oxide may be
lighted.    In another fifteen minutes, the gas which is passing
through the tube may be tested.    The current is allowed to                     t   ^
slow down a little, and the graduated tube of the azotometer is                   -     
then  filled with potash solution by raising the reservoir and
closing the tap.    On gradually lowering the  reservoir, a  few
bubbles will pass up the graduated tube.
By the time they reach the top of the tube, the size of the
bubbles should have become so minute that when collected at the
top they occupy no appreciable volume, but appear as a fine froth.
If this is not the case, open the tap, run out the solution and
continue as before to drive carbon dioxide through the tube.
Repeat the test in another five minutes. Not more than half
the bicarbonate should have been utilised in expelling the air.
The air being removed, the combustion of the substance is com-
menced. The azotometer is filled with the potash solution, the
tap closed, and the reservoir lowered as far as possible. The
current of carbon 'dioxide is allowed to slacken, but it must not
be completely stopped. The front portion of the combustion
tube will by this time have reached a dull red heat. A few
more burners are now lighted on both sides of the fine oxide.
Finally, the layer of fine oxide is gradually heated and the pro-
cess conducted in much the same manner as that described
under the estimation of carbon and hydrogen. The combustion
is regulated by the speed of the bubbles passing up the
azotometer tube, which should enable them to be readily
counted. The burners being all lighted and the tube red hot
throughout, the tiles above the substance are closed. The
current of gas will shortly slacken. The residual nitrogen is
then expelled from the tube by moving on the flame beneath
the bicarbonate and causing a fresh stream of carbon dioxide to
sweep through the tube. Care must be taken that the stream
of gas is not too rapid, as otherwise the potash solution may
become saturated and driven completely into the reservoir. The                  ' 
burners may now be extinguished and a reading of the level in
the azotometer taken every few minutes until it remains constant
and the bubbles are completely absorbed. Remove the
COHEN'S ADV. p. o. c.                                             c